linux-kernel - Re: [PATCH 06/10] sched/fair: Clear the target CPU from the cpumask of CPUs searched

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Date:   Fri, 4 Dec 2020 16:23:48 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Ziljstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Linux-ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 06/10] sched/fair: Clear the target CPU from the cpumask
 of CPUs searched

On Fri, 4 Dec 2020 at 15:31, Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> On Fri, Dec 04, 2020 at 02:47:48PM +0100, Vincent Guittot wrote:
> > > IIUC, select_idle_core and select_idle_cpu share the same cpumask(select_idle_mask)?
> > > If the target's sibling is removed from select_idle_mask from select_idle_core(),
> > > select_idle_cpu() will lose the chance to pick it up?
> >
> > This is only relevant for patch 10 which is not to be included IIUC
> > what mel said in cover letter : "Patches 9 and 10 are stupid in the
> > context of this series."
> >
>
> Patch 10 was stupid in the context of the prototype because
> select_idle_core always returned a CPU. A variation ended up being
> reintroduced at the end of the Series Yet To Be Posted so that SMT siblings
> are cleared during select_idle_core() but select_idle_cpu() still has a
> mask with unvisited CPUs to consider if no idle cores are found.
>
> As far as I know, this would still be compatible with Aubrey's idle
> cpu mask as long as it's visited and cleared between select_idle_core
> and select_idle_cpu. It relaxes the contraints on Aubrey to some extent
> because the idle cpu mask would be a hint so if the information is out
> of date, an idle cpu may still be found the normal way.

But even without patch 10, just replacing sched_domain_span(sd) by
sds_idle_cpus(sd->shared) will ensure that sis loops only on cpus that
get a chance to be idle so select_idle_core is likely to return an
idle_candidate

>
> --
> Mel Gorman
> SUSE Labs